There is a wide variety of commercially-available displacement measurement devices, such as optical rulers, potentiometers, and laser rangefinders (precise but expensive), to choose from as needed in terms of measurement requirement, precision, and surroundings.
Operating by optical theories, an optical ruler essentially comprises a grating ruler, a mask plate, a plurality of light-emitting devices, and a plurality of sensing devices. Both the grating ruler and the mask plate consist of transparent and opaque lines spaced apart and distributed uniformly. The transparent and opaque lines of the grating ruler are spaced apart equidistantly. The lines of mask plates are distributed in a way to make the mask plates fall substantially into two categories: mask plate A and mask plate B. There is a 90° angle difference between the two mask plates. Therefore, when transparent and opaque lines of mask plate A and grating ruler are arranged neatly, transparent and opaque lines of mask plate B and grating ruler differ by 0.25 grating cycle, and vice versa. The light-emitting devices in the optical ruler are disposed on the back of the grating ruler, whereas the sensing devices are disposed on the back of mask plate A and mask plate B. With the optical ruler being in operation, both the grating ruler and a light emitter are fixed in place, using the moving mask plates and sensor to measure displacement. The conventional optical ruler is enclosed by a metallic casing. By contrast, novel optical rulers dispense with any metallic casing but are each equipped with a sliding block and a magnetic ruler body so as to operate in the same way as conventional optical rulers, thereby being advantageously lightweight. However, the novel optical rulers are disadvantaged by a complicated process of adhering the ruler body in place. As a result, the novel optical rulers have to be adhered in place by technicians.
Depending on an adopted sensing principle, potentiometers fall into three categories: non-contact magnetic induction potentiometers, non-contact magnetoresistance variable potentiometers, and contact-style electrically conductive rubber potentiometers.
The non-contact magnetic induction potentiometer comprises a Hall device and a permanent magnet. A magnetic field variation produced by the displacement between the permanent magnet and a bias winding is converted into an electrical signal by the Hall device.
The non-contact magnetoresistance variable potentiometer comprises a magnetic induction resistance device and a permanent magnet. The permanent magnet is fixed in place, and the magnetic induction resistance device undergoes a displacement to therefore produce an impedance variation, so that median voltage output is measured by voltage division, thereby calculating the displacement.
The contact-style electrically conductive rubber potentiometer is integrally formed by pressing and heating the surface of synthetic resin and a refined carbon element fine filler, wherein onto its smooth sliding surface carbon elements are printed. Its manufacturing process involves adjusting local resistance by laser cutting calibration so that it is capable of precise voltage—position performance.
However, the aforesaid displacement measurement devices have a disadvantage in common: after being purchased, the displacement measurement devices have their journey specified; to increase or decrease the journey, the owner has to buy another displacement system, not to mention that a slide with a long journey takes up too much space, and in consequence the displacement measurement devices are not portable.